Many particle sorting or separation schemes exist, ranging from gel-electrophoresis, capillary electrophoresis, and analytical centrifuging to novel, entropic barriers. Examples of these are described by J. Han, H. G. Craighead, Science 288, 1026-1029 (May 12, 2000) and D. Nykypanchuk, H. H. Strey, D. A. Hoagland, Science 297, 987-990 (Aug. 9, 2002). The majority of these known techniques separate a polydisperse mixture in a flowing fluid into bands containing particles that travel at different velocities along the direction of flow. This typically leads to batch processing. In electrophoresis a gel is used to obtain a size-dependent mobility. Recovery of fractions is achieved through post-processing of the gel. However, despite its widespread use and effectiveness this methodology is slow and importantly, due to limited pore sizes, has difficulty in separating objects at the microscopic size level, for example cells, chromosomes and colloidal matter.
Lithographically fabricated two-dimensional, asymmetric artificial gels are also used. Examples of these are described by D. Ertas, Physical Review Letters 80, 1548-1551 (Feb. 16, 1998); T. A. I Duke, R. H. Austin, Physical Review Letters 80, 1552-1555 (Feb. 16, 1998) and C. F. Chou et al., Biophysical Journal 83, 2170-2179 (October 2002). These gels yield separation transverse to the direction of flow. Because of this, they can be operated in a continuous fashion, with various fractions taken up by separate collection channels. However, sorting based on diffusion is impractically slow at the microscopic scale.
In recent years there has been growth in the exploration of particle motion on optical landscapes. An example of this is described in the article “Kinetically Locked-in Colloidal Transport in an Array of Optical Tweezers” by P. T. Korda et al, Physical Review Letters 89, Number 12, Art. No. 128301 (16 Sep. 2002). In this case, a monolayer of colloidal spheres is allowed to flow through an array of discrete optical traps. By varying the orientation of the array of traps, the direction of flow of the spheres can be varied. Because of this, it has been suggested that the lattice could be used to continuously fractionate mesoscopic particles. However, because of the use of a lattice of localized discrete traps, the observed kinetically locked-in channelling along low-index lattice vectors is intrinsically limited to small-angle deflections. In practice, this limits the practicality of the lattice for use in fractionation.
PCT/GB2004/001993 describes yet another optical fractionation scheme. In this, three-dimensional optical lattices are used for sorting and fractionation of biological and colloidal material in a microfluidic flow. Different particles follow different trajectories across the landscape and consequently exit at different points. The selectivity and basis of this form of sorting is the affinity of a given particles to the features of the optical landscape. This is also described by M. MacDonald, G. Spalding and K. Dholakia, in Nature 426, 421 (2003), and by A. M. Lacasta, et al., in Physical Review Letters (2005), 94, 188902. Even in the absence of fluid flow periodic optical patterns may be used for sorting, see L. Paterson, et al., Applied Physics Letters (2005), 87, 123901.
One of the main advantages of using optically defined microfluidic sorting is that the requirements on the physical microfluidics can be kept to a minimum. Nevertheless, in some circumstances there is a need for an even simpler arrangement.